The proposed research plan is essentially designed to determine how sensory inputs are utilized for the control of motor outputs. The intentional initiation of movements and the automatic control of ongoing movements (particularly by the cerebellum) will be considered. Human subjects (both normal and cerebellar patients) and trained monkeys (before and after discrete central lesions) will be used. Essentially, two motor tasks will be used: tracking and ballistically initiated movements. Perturbation will be introduced in the course of these movement, by using a torque motor, to test the behavior of the stretch reflex. The hope is to define the contribution by different subsystems in the control of motion and the logic of such control. In particular, we hope to be able to test independently the excitability of the alpha motoneuron pool ( in the agonist and antagonist) and the changes in gain of the gamma loop (therefore including the spindle sensitivity). If so, we will be able to prove or disprove specific hypotheses presented in detail in this proposal. Finally, an attempt will be made to characterize phenomenologically the disturbance in the control of posture resulting from labyrinthine and cerebellar lesions.